How to clean the hard drive before selling? Forgot password to login

We have an old iMac from the early 2000s (like the one pictured below). We staked it away all these years and now decided to sell it. The problem is, that we do not recall the two passwords for the two users (my husband and I). We would like to delete the data that is stored and all history and reset it to factory settings before selling it. We have yet to figure out a way to do this. Any input?